Can't send text messages on my unlocked blackberry storm 2

Hi everyone! I recently bought a unlocked blackberry storm 2 online and I put my AT&T SIM card in it. It had a few problems that I figured out myself but now I'm having problems with text messages . I can receive them but can not reply or compose my own SMS. If someone knows what the problem is and can help me It would be greatly appreciated. Thank you!

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